Synthetic amphiphilic macromolecules composed of stearic acid covalently linked to fluorescent dextran derivatives were synthesized as models for membrane proteins. The water-soluble molecules adsorb to all artificial and cell membranes tested. In the past year we have principally studied the lateral diffusion and qualitative behavior of these molecules on the surface of fibroblasts. The lateral diffusion constant is 2x10 to the minus 10th power cm2/sec., a rate typical of membrane proteins. Addition of antibody to these dextrans causes patching and capping of these molecules. The lateral diffusion of these molecules is reduced by antibody crosslinking at lower concentrations than gives rise to visible patching. In the case of very flat fibroblasts these absorbed membrane molecules form patches in rows as has been observed for natural membrane antigens. On lymphocytes, the study of capping has been extended to thymocytes which show quantitative differences from splenocytes with respect to kinetics and dose dependence, but show a similar drug inhibition profile. In addition the technology developed was used to allow the formation of IgA coated erthrocytes which have successfully detected IgA specific Fc receptors on mouse lymphocytes.